Aluminum hydroxide and production process thereof

ABSTRACT

A method for producing aluminum hydroxide includes a step of elevating a temperature of a slurry of aluminum hydroxide suspended in a sodium aluminate solution from 70° C. or lower to 85° C. or higher, and a step of applying a centrifugal force to the slurry. The aluminum hydroxide constituting the slurry is obtained beforehand through the Bayer&#39;s process.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is an application filed under 35 U.S.C. § 111(a)claiming the benefit pursuant to 35 U.S.C. § 119(e) (1) of the filingdate of U.S. Provisional Application No. 60/300,852 filed Jun. 27, 2001pursuant to 35 U.S.C. §111(b).

TECHNICAL FIELD

The present invention relates to aluminum hydroxide that is employed asflame-retardant filler or a similar material for use in plastics,rubber, etc., and to a method for producing the aluminum hydroxide. Moreparticularly, the invention relates to aluminum hydroxide that has beendisintegrated to thereby form separate, individual particles withoutcausing unfavorable phenomena, such as roughening of particle surfacesand chipping of the particles, and to a method for producing thealuminum hydroxide.

BACKGROUND ART

Conventionally, aluminum hydroxide has been employed as filler forfulfilling various functions in rubber/plastics. For example, aluminumhydroxide is added, as a flame retardant, to thermoplastic resins,rubber or epoxy resins, and is added, as color-controlling filler, tothermosetting resins, such as unsaturated polyester resins and acrylicresins.

When aluminum hydroxide is employed as a flame retardant, flameretardation performance is enhanced as the amount of aluminum hydroxideis increased. However, addition of a large amount of aluminum hydroxideelevates the torque required for kneading and the molding temperature,thereby causing problematic foaming due to dehydration of a portion ofaluminum hydroxide. When aluminum hydroxide is added to thermosettingresins, an increase in the amount of the added aluminum hydroxidereduces material costs, but material strength problematically decreases.

In order to prevent decrease in material strength, particle size isdesirably reduced to be as small as possible. Although aluminumhydroxide particles having a small particle size can be yielded throughprecipitation, addition of a large amount of such small particlesserving as filler is difficult, since aluminum hydroxide formsagglomerated secondary particles, which are formed through agglomerationof a large number of primary particles, and exhibits considerably highabsorption of oil. Accordingly, aluminum hydroxide particles having aparticle size of approximately 50 to 150 μm are pulverized by means of aball mill or other pulverizer to thereby form approximately primaryparticles, which are generally employed as filler.

However, pulverizing the particles to a predetermined particle sizethrough a pulverization technique requires a large amount of energy. Inaddition, aluminum hydroxide primary particles yielded throughpulverization are caused to break, thereby causing roughening ofparticle surfaces, chipping of the particles, etc. As a result, the BETspecific surface area of the resultant powder increases. Thus, suchpowder has poor compatibility with resin and increases the viscosity ofthe resin containing the powder, failing to attain high-densityincorporation of the powder. When the powder is added to a thermosettingresin, the curing time of the resin is prolonged.

On the basis of these tendencies, aluminum hydroxide ideal for servingas filler is thought to have a small surface roughness, i.e., a smallBET specific surface area, and to form separate, individual particles.

JP-B HEI 5-4336 discloses a method for disintegrating agglomeratedsecondary particles by application of strong centrifugal force by meansof a continuous screw decanter without breaking the correspondingprimary particles, thereby preventing roughening of surfaces of theprimary particles. However, the above method is limited to applicationsof a specific raw material. That is, the method cannot be applied to awide range of materials.

JP-B SHO 62-9256 discloses a method for producing single-crystalline orroundish aluminum hydroxide particles by bringing a temperature-elevatedBayer extract into contact with solid aluminum hydroxide. However, theabove method has drawbacks in that it requires a long period of contacttime and that dissolution of aluminum hydroxide during contact ispromoted, thereby deteriorating production efficiency.

JP-A HEI 9-208740 discloses a method for reducing the BET specificsurface area of aluminum hydroxide particles by pulverizing in advanceagglomerated aluminum hydroxide secondary particles by means of adry-impact pulverizer, adding the pulverized product into a sodiumaluminate solution having a predetermined alkaline concentration tothereby form a slurry, and elevating the temperature of the slurry tothereby dissolve the particle surfaces. However, the method also has adrawback in that aluminum hydroxide has to be filtered and dried inorder to effect dry pulverization performed in advance, therebyprolonging production steps and elevating production costs.

Accordingly, an object of the present invention is to provide aluminumhydroxide ideal for serving as filler, the aluminum hydroxide having asmall BET specific surface area over a wide particle size range andforming separate, individual particles.

Another object of the invention is to provide a method for effectivelyproducing the aluminum hydroxide.

In view of the foregoing, the present inventors have carried outextensive studies in order to attain the above objects, and have foundthat ideal aluminum hydroxide having a small specific surface areasuitable for serving as filler and forming separate, individualparticles can be produced through a combination of a first step ofelevating the temperature of a slurry under predetermined conditions,which slurry is yielded by suspending aluminum hydroxide in a specificsodium aluminate solution, and a second step of applying a centrifugalforce to the slurry to thereby form a sediment having an elevated solidcontent and disintegrate agglomerated secondary particles. The presentinvention has been accomplished on the basis of this finding.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention provides a method for producingaluminum hydroxide comprising a step of elevating a temperature of aslurry of aluminum hydroxide suspended in a sodium aluminate solutionfrom 70° C. or lower to 85° C. or higher, the aluminum hydroxide havingbeen obtained through the Bayer's process, and a step of applying acentrifugal force to the slurry.

The present invention further provides a method for producing aluminumhydroxide comprising elevating a temperature of a slurry of aluminumhydroxide suspended in a sodium aluminate solution from 70° C. or lowerto 85° C. or higher, the aluminum hydroxide having been obtained throughthe Bayer's process, and subsequently applying a centrifugal force tothe slurry.

In each of the methods, the slurry temperature elevation is conductedwithin 15 minutes.

In any one of the methods, the sodium aluminate solution has a ratio A/Cof an alumina concentration A (g/liter) to a sodium hydroxideconcentration C (g/liter) of 0.45 or less.

In any one of the methods, the aluminum hydroxide has a percentdissolution, due to the slurry temperature elevation, of less than 15%,the percent dissolution being represented by the following formula:Percent dissolution (%)=C (before temperature elevation)×{A/C (aftertemperature elevation)−A/C (before temperature elevation)}×1.53/slurryconcentration (before temperature elevation)×100, wherein A representsthe alumina concentration (g/liter) of the sodium aluminate solution andC represents the sodium hydroxide concentration (g/liter) of the sodiumaluminate solution.

In any one of the methods, the centrifugal force is at least 300 G.

In any one of the methods, the centrifugal force is applied by means ofa continuous screw decanter.

In any one of the methods, the slurry temperature elevation is performedusing a double-tube heat exchanger serving as a temperature elevationapparatus employed in the step for elevating the temperature.

The present invention also provides aluminum hydroxide produced throughany one of the methods described above, which has a mean particle size Dof 1 to 25 μm; a BET specific surface area S of 1.5 m²/g or less; and aratio D/D_(bet) (agglomeration degree) of D to a particle size D of lessthan 3, the particle size D_(bet) being a sphere-equivalent particlesize calculated on the basis of D_(bet)=6/(s×ρ), wherein ρ representsthe density of aluminum hydroxide.

The present invention also provides an aluminum hydroxide compositioncomprising, as filler, the aluminum hydroxide just mentioned above.

The aluminum hydroxide composition comprises a matrix material of rubberor plastic.

As described above, by applying thermal impact to aluminum hydroxide andselectively affecting the grain boundary of the agglomerated secondaryparticles by crystallographically weak cohesive force, it is possible toobtain aluminum hydroxide having a small specific surface area andforming separate, individual particles, which is suitable for filler.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will next be described in more detail.

The present invention provides a method for producing aluminum hydroxidecomprising a step for elevating the temperature of a slurry of aluminumhydroxide suspended in a sodium aluminate solution from 70° C. or lowerto 85° C. or higher, the aluminum hydroxide having been obtained throughthe Bayer's process, and a step for applying a centrifugal force to theslurry.

The present invention also provides a method for producing aluminumhydroxide comprising elevating the temperature of a slurry of aluminumhydroxide suspended in a sodium aluminate solution from 70° C. or lowerto 85° C. or higher, the aluminum hydroxide having been obtained throughthe Bayer's process, and subsequently applying a centrifugal force tothe slurry.

In other words, the present invention is directed to a method forproducing aluminum hydroxide comprising a first step and a second stepin combination. In the first step of elevating the temperature of aslurry of aluminum hydroxide (obtained through the Bayer's process)suspended in a sodium aluminate solution from 70° C. or lower to 85° C.or higher, thermal impact is applied to the aluminum hydroxide, therebyselectively affecting interfaces between the primary particles thatconstitute agglomerated secondary particles (hereinafter referred tosimply as particle interfaces), which are agglomerated bycrystallographically weak cohesive force, to thereby disintegrate theagglomerated secondary particles. In the second step, a centrifugalforce of preferably at least 300 G is applied to the slurry to therebydisintegrate the agglomerated secondary particles by attrition betweenparticles in forming sediment having an elevated solid content and byshearing force exerted onto the solid content in continuously scrapingthe solid content out of the centrifuge.

The above method can provide aluminum hydroxide having a mean particlesize D of 1 to 25 μm; a specific surface area S, as measured through anitrogen absorption method (BET method), of 1.5 m²/g or less; and aratio D/D_(bet) (agglomeration degree) of D to a particle size D_(bet)of less than 3, the particle size D_(bet) being a sphere-equivalentparticle size calculated from S, i.e., D_(bet)=6/(S×ρ), wherein ρrepresents the density of aluminum hydroxide.

In the method for producing aluminum hydroxide according to the presentinvention, the period of time during which the temperature is elevatedfrom 70° C. or lower to 85° C. or higher is 15 minutes or shorter,preferably 10 minutes or shorter, more preferably 5 minutes or shorter.Periods of time of longer than 15 minutes are not preferred, sincethermal impact required for selectively affecting particle interfacescannot be provided, and the entirety of the particles is dissolved.

The sodium aluminate solution employed in the present method has a ratioA/C of an alumina (Al₂O₃) concentration A (g/liter) to a sodiumhydroxide (NaOH) concentration C (g/liter) of 0.45 or less, preferably0.40 or less, more preferably 0.35 or less. A/C ratio values of higherthan 0.45 are not preferred, since aluminum hydroxide in an amountrequired for affecting particle interfaces through temperature elevationcannot be dissolved.

The percent dissolution of aluminum hydroxide due to temperatureelevation in the present method is less than 15%, preferably less than13%. When the percent dissolution is more than 15%, yield of aluminumhydroxide decreases, thereby deteriorating production efficiency.

The percent dissolution of aluminum hydroxide due to temperatureelevation is defined by the following formula:Percent dissolution (%)=C (before temperature elevation)×{A/C (aftertemperature elevation)−A/C (before temperature elevation)}×1.53/slurryconcentration (before temperature elevation)×100, wherein A representsthe alumina concentration (g/liter) of the sodium aluminate solution andC represents the sodium hydroxide concentration (g/liter) of the sodiumaluminate solution.

In the present method, temperature is elevated to at least 85° C.,preferably at least 90° C., more preferably at least 95° C. When thetemperature is lower than 85° C., thermal impact required forselectively affecting particle interfaces cannot be provided, andtherefore such a low temperature is not preferred, since dissolution ofparticles progresses from the surfaces of the particles, therebydeteriorating production efficiency, and the particle surfaces areroughened.

Although the boiling point of a sodium aluminate solution is notconstant and varies depending on the sodium hydroxide concentrationthereof, a sodium aluminate solution employed in a step of the Bayer'sprocess has a boiling point of approximately 104° C.

In the present method, the temperature of the slurry before the slurryundergoes temperature elevation is 70° C. or lower, preferably 65° C. orlower. Slurry temperatures of higher than 70° C. are not preferred,since thermal impact required for selectively affecting particleinterfaces cannot be provided in the course of temperature elevation.

JP-B HEI 5-4336 discloses a method for disintegrating agglomeratedsecondary particles by application of strong centrifugal force by meansof a continuous screw decanter without breaking the correspondingprimary particles. This technique is considered to provide the effect ofdisintegrating agglomerated secondary particles on the basis of themechanism that the agglomerated secondary particles collide one anotherwhile moving under centrifugal force applied to the aluminum hydroxideslurry, thereby causing attrition among the particles. This method isdescribed to be applicable only to raw material aluminum hydroxidehaving a primary particle size of 1 to 4 μm, and it is described thataluminum hydroxide particles having a primary particle size of 4 μm ormore cannot sufficiently enjoy the effect of disintegrating agglomeratedsecondary particles exerted by centrifugal force.

However, through combination of the disintegrating method by means of acontinuous screw decanter and the aforementioned step of disintegratingagglomerated secondary particles of aluminum hydroxide includingelevating the temperature of the aforementioned slurry of aluminumhydroxide suspended in a sodium aluminate solution, aluminum hydroxidehaving a primary particle size of more than 4 μm can be disintegrated toform separate, individual particles without roughening the particlesurfaces. Such aluminum hydroxide particles have never beensatisfactorily disintegrated by the method disclosed in JP-B HEI 5-4336.The centrifugal force applied in the present method is at least 300 G,preferably at least 500 G. more preferably at least 1,000 G. Acentrifugal force of less than 300 G is not preferred, because it isinsufficient for disintegrating agglomerated secondary particles.

In the present invention, aluminum hydroxide particles are disintegratedalso by shearing force exerted onto an elevated solid content insediment when continuously scraping the solid content out of thecentrifuge.

In the present invention, aluminum hydroxide that has a small BETspecific surface area at a desired particle size and forms separate,individual particles can be produced by selecting the primary particlesize of agglomerated secondary particles serving as raw material inconsideration of the desired particle size realized through dissolution.

The aluminum hydroxide produced according to the present invention issuitably employed as filler for fulfilling various functions. Examplesof preferably used matrix material to which a composition containingaluminum hydroxide serving as filler is added include rubber andplastics, such as thermoplastic resins, epoxy resins and thermosettingresins (e.g., unsaturated polyester resins and acrylic resins).

When added to a resin or a similar material, the aluminum hydroxideproduced according to the present method may be used singly or incombination of several types of aluminum hydroxide particles havingdifferent particle sizes so as to reduce the compound viscosity.

The aluminum hydroxide produced according to the present method may betreated with a conventionally known surface-treating agent. Noparticular limitation is imposed on the surface-treating agent, andexamples include coupling agents, such as silane coupling agents andtitanate coupling agents; fatty acids, such as oleic acid and stearicacid; esters thereof; and silicates, such as methyl silicate and ethylsilicate.

The present invention will next be described in more detail usingexamples, which should not be construed as limiting the inventionthereto. In the present invention, physical properties were measuredthrough the following methods.

The mean particle size D of aluminum hydroxide was measured through alaser scattering diffraction method.

The specific surface area S of aluminum hydroxide was measured through anitrogen absorption method (BET method).

The agglomeration degree of aluminum hydroxide was estimated on thebasis of the ratio D/D_(bet) of D to the particle size D_(bet), which isa sphere-equivalent particle size calculated on the basis ofD_(bet)=6/(S×ρ), wherein ρ represents the density of aluminum hydroxide.

The percent dissolution of aluminum hydroxide due to temperatureelevation was calculated on the basis of the following formula:Percent dissolution (%)=C (before temperature elevation)×{A/C (aftertemperature elevation)−A/C (before temperature elevation)}×1.53/slurryconcentration (before temperature elevation)×100, wherein A representsthe alumina concentration (g/liter) of the sodium aluminate solution andC represents the sodium hydroxide concentration (g/liter) of the sodiumaluminate solution.

EXAMPLE 1

A slurry of aluminum hydroxide which had been produced through theBayer's process (mean particle size of aluminum hydroxide: 93.3 μm,sodium hydroxide concentration: 150 g/liter, A/C=0.33, slurryconcentration: 220 g/liter, and slurry temperature: 34° C.) was fed tothe inner tube of a double-tube heat exchanger (inner tube capacity:0.019 m³, heat transfer area: 3.2 m²) at 3 m³/hr (residence time in theheat exchanger: 23 seconds). By feeding steam to the outer tube of theheat exchanger, the temperature of the slurry was elevated to 96° C.,and a portion of the slurry was fed to a continuous screw decanter(SHARPES SUPER-D-CANTER P-660, product of Tomoe engineering) at 1 m³/hr,so as to apply a centrifugal force (1,000 G) to the slurry to therebyform a sediment having an elevated solid content. The solid content wascontinuously taken out of the centrifuge using a screw mounted on theinner wall of the centrifuge.

The A/C and percent dissolution of the slurry that had been passedthrough the double-tube heat exchange were found to be 0.44 and 11.5%,respectively. The aluminum hydroxide that had been passed through thedecanter was subjected to washing, separating through filtration, anddrying. The thus-yielded aluminum hydroxide was found to have a meanparticle size D of 20.0 μm, a BET specific surface area S of 0.3 m²/g,and an agglomeration degree of 2.4.

EXAMPLE 2

A slurry of aluminum hydroxide which had been produced through theBayer's process (mean particle size of aluminum hydroxide: 76.8 μm,sodium hydroxide concentration: 146 g/liter, A/C=0.38, slurryconcentration: 190 g/liter, and slurry temperature: 65° C.) was fed tothe inner tube of a double-tube heat exchanger similar to that employedin Example 1 at 3 m³/hr (residence time in the heat exchanger: 23seconds). By feeding steam to the outer tube of the heat exchanger, thetemperature of the slurry was elevated to 96° C., and a portion of theslurry was fed to a continuous screw decanter similar to that employedin Example 1 at 1 m³/hr, so as to apply a centrifugal force (1,000 G) tothe slurry to thereby form a sediment having an elevated solid content.The solid content was taken out of the centrifuge in the same manner asin Example 1.

The A/C and percent dissolution of the slurry that had been passedthrough the double-tube heat exchange were found to be 0.49 and 12.9%,respectively. The aluminum hydroxide that had been passed through thedecanter was subjected to washing, separating through filtration, anddrying. The thus-yielded aluminum hydroxide was found to have a meanparticle size D of 12.4 μm, a BET specific surface area S of 0.4 m²/g,and an agglomeration degree of 2.0.

EXAMPLE 3

A slurry of aluminum hydroxide which had been produced through theBayer's process (mean particle size of aluminum hydroxide: 20.8 μm,sodium hydroxide concentration: 154 g/liter, A/C=0.35, slurryconcentration: 230 g/liter, and slurry temperature: 64° C.) was fed tothe inner tube of a double-tube heat exchanger similar to that employedin Example 1 at 3 m³/hr (residence time in the heat exchanger: 23seconds). By feeding steam to the outer tube of the heat exchanger, thetemperature of the slurry was elevated to 97° C., and a portion of theslurry was fed to a continuous screw decanter similar to that employedin Example 1 at 1 m³/hr, so as to apply a centrifugal force (1,000 G) tothe slurry to thereby form a sediment having an elevated solid content.The solid content was taken out of the centrifuge in the same manner asin Example 1.

The A/C and percent dissolution of the slurry that had been passedthrough the double-tube heat exchange were found to be 0.46 and 11.3%,respectively. The aluminum hydroxide that had been passed through thedecanter was subjected to washing, separating through filtration, anddrying. The thus-yielded aluminum hydroxide was found to have a meanparticle size D of 3.3 μm, a BET specific surface area S of 1.3 m²/g,and an agglomeration degree of 1.7.

EXAMPLE 4

A portion of the slurry of Example 2 which had been passed through adouble-tube heat exchange was fed to a continuous screw decanter similarto that employed in Example 2 at 1 m³/hr, so as to apply a centrifugalforce (500 G) to the slurry to thereby form a sediment having anelevated solid content. The solid content was taken out of thecentrifuge in the same manner as in Example 1.

The aluminum hydroxide that had been passed through the decanter wassubjected to washing, separating through filtration, and drying. Thethus-yielded aluminum hydroxide was found to have a mean particle size Dof 13.6 μm, a BET specific surface area S of 0.4 m²/g, and anagglomeration degree of 2.2.

COMPARATIVE EXAMPLE 1

Slurry of aluminum hydroxide similar to that employed in Example 1 wasfed to the inner tube of a double-tube heat exchanger similar to thatemployed in Example 1 at 3 m³/hr (residence time in the heat exchanger:23 seconds). By feeding steam to the outer tube of the heat exchanger,the temperature of the slurry was elevated to 83° C., and a portion ofthe slurry was fed to a continuous screw decanter (product of Tomoeengineering) similar to that employed in Example 1 at 1 m³/hr, so as toapply a centrifugal force (1,000 G) to the slurry to thereby form asediment having an elevated solid content. The solid content was takenout of the centrifuge in the same manner as in Example 1.

The A/C and percent dissolution of the slurry that had been passedthrough the double-tube heat exchange were found to be 0.35 and 2.1%,respectively. The aluminum hydroxide that had been passed through thedecanter was subjected to washing, separating through filtration, anddrying. The thus-yielded aluminum hydroxide was found to have a meanparticle size D of 80.6 μm, a BET specific surface area S of 0.3 m²/g,and an agglomeration degree of 9.8.

COMPARATIVE EXAMPLE 2

Slurry of aluminum hydroxide which had been produced through the Bayer'sprocess (mean particle size of aluminum hydroxide: 93.3 μm, sodiumhydroxide concentration: 147 g/liter, A/C=0.47, slurry concentration:210 g/liter, and slurry temperature: 63° C.) was fed to the inner tubeof a double-tube heat exchanger similar to that employed in Example 1 at3 m³/hr (residence time in the heat exchanger: 23 seconds). By feedingsteam to the outer tube of the heat exchanger, the temperature of theslurry was elevated to 95° C., and a portion of the slurry was fed to acontinuous screw decanter similar to that employed in Example 1 at 1m³/hr, so as to apply a centrifugal force (1,000 G) to the slurry tothereby form a sediment having an elevated solid content. The solidcontent was taken out of the centrifuge in the same manner as in Example1.

The A/C and percent dissolution of the slurry that had been passedthrough the double-tube heat exchange were found to be 0.49 and 2.1%,respectively. The aluminum hydroxide that had been passed through thedecanter was subjected to washing, separating through filtration, anddrying. The thus-yielded aluminum hydroxide was found to have a meanparticle size D of 24.5 μm, a BET specific surface area S of 0.4 m²/g,and an agglomeration degree of 4.0.

COMPARATIVE EXAMPLE 3

A portion of the slurry of Example 1 which had been passed through adouble-tube heat exchange was fed to a continuous screw decanter similarto that employed in Example 1 at 1 m³/hr, so as to apply a centrifugalforce (100 G) to the slurry to thereby form a sediment having anelevated solid content. The solid content was taken out of thecentrifuge in the same manner as in Example 1.

The aluminum hydroxide that had been passed through the decanter wassubjected to washing, separating through filtration, and drying. Thethus-yielded aluminum hydroxide was found to have a mean particle size Dof 30.7 μm, a BET specific surface area S of 0.3 m²/g, and anagglomeration degree of 3.7.

COMPARATIVE EXAMPLE 4

Aluminum hydroxide slurry similar to that of Example 1 was fed to a SUStank (capacity: 1 m³), and the temperature of the tank was elevated to85° C. over 30 minutes. Subsequently, the heated slurry was fed to acontinuous screw decanter similar Lo that employed in Example 1 at 1m³/hr, so as to apply a centrifugal force (1,000 G) to the slurry tothereby form a sediment having an elevated solid content. The solidcontent was taken out of the centrifuge in the same manner as in Example1.

The A/C and percent dissolution of the slurry contained in the tank,after temperature elevation, were found to be 0.52 and 19.8%,respectively. The aluminum hydroxide that had been passed through thedecanter was subjected to washing, separating through filtration, anddrying. The thus-yielded aluminum hydroxide was found to have a meanparticle size D of 19.8 μm, a BET specific surface area S of 0.3 m²/g,and an agglomeration degree of 2.4.

The results of Examples 1 to 4 and Comparative Examples 1 to 4 aresummarized in Table 1 below.

TABLE 1 (1/2) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Al hydroxide raw materialcharacteristics Mean particle size μm 93.3 76.8 20.8 76.8 Na aluminatesolution characteristics NaOH concentration g/L 150 146 154 146 A/C -0.33 0.38 0.35 0.38 Slurry characteristics Slurry concentration g/L 220190 230 190 Slurry temperature ° C. 34 65 64 65 Temp. elevationconditions Temp. elevated ° C. 96 97 96 96 Temp. elevation time 23 sec23 sec 23 sec 23 sec A/C (after temp. elevation) - 0.44 0.49 0.46 0.49Percent dissolution % 11.5 12.9 11.3 12.9 Centrifugation conditionsCentrifugal force G 1000 1000 1000 500 Al hydroxide characteristics(after filtration) Mean particle size D μm 20.0 12.4 3.3 13.6 BET sp.surface area S m²/g 0.3 0.4 1.3 0.4 Agglomeration degree 2.4 2.0 1.7 2.2

TABLE 1 (2/2) Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Alhydroxide raw material characteristics Mean particle size μm 93.3 93.393.3 93.3 Na aluminate solution characteristics NaOH concentration g/L150 147 150 150 A/C - 0.33 0.47 0.33 0.33 Slurry characteristics Slurryconcentration g/L 220 210 220 220 Slurry temperature ° C. 34 63 34 34Temp. elevation conditions Temp. elevated ° C. 83 95 96 85 Temp.elevation time 23 sec 23 sec 23 sec 30 min A/C (after temp. elevation) -0.35 0.49 0.44 0.52 Percent dissolution % 2.1 2.1 11.5 19.8Centrifugation conditions Centrifugal force G 1000 1000 100 1000 Alhydroxide characteristics (after filtration) Mean particle size D μm80.6 24.5 30.7 19.8 BET sp. surface area S m²/g 0.3 0.4 0.3 0.3Agglomeration degree 9.8 4.0 3.7 2.4

INDUSTRIAL APPLICABILITY

According to the production method of the present invention,disintegrating of agglomerated secondary particles of aluminum hydroxideis performed by applying thermal impact to aluminum hydroxide;selectively affecting interfaces between the primary particles thatconstitute agglomerated secondary particles, which are agglomerated bycrystallographically weak cohesive force; and applying centrifugal forceso as to attain strong contact between agglomerated secondary particles.

As described hereinabove, differing from conventionally employedpulverization methods making use of impact force generated by attritionof media; grinding methods making use of a mill, such as a Raymond mill;and pulverization methods making use of collision of particles (e.g.,jet mill), the method for producing aluminum hydroxide of the presentinvention is an epoch-making method in that the primary particlesurfaces are not roughened during the course of disintegrating. Thus,the aluminum hydroxide produced through the method of the presentinvention is suitable for filler and is of remarkably great industrialvalue, since the aluminum hydroxide has a small specific surface areaand forms separate, individual particles.

1. A method for producing aluminum hydroxide comprising: a step ofelevating a temperature of a slurry having agglomerated secondaryparticles of aluminum hydroxide obtained through the Bayer's processsuspended in a sodium aluminate solution from 70° C. or lower to 85° C.or higher within 15 minutes to disintegrate agglomerated secondaryparticles, and a step of applying a centrifugal force to the slurry todisintegrate agglomerated secondary particles by attrition betweenparticles that are formed in a sediment having an elevated solidcontent.
 2. A method for producing aluminum hydroxide comprising:elevating a temperature of a slurry having agglomerated secondaryparticles of aluminum hydroxide obtained through the Bayer's processsuspended in a sodium aluminate solution from 70° C. or lower to 85° C.or higher within 15 minutes to disintegrate agglomerated secondaryparticles, and subsequently applying a centrifugal force to the slurryto disintegrate agglomerated secondary particles by attrition betweenparticles that are formed in a sediment having an elevated solidcontent.
 3. The method according to any one of claims 1 or 2, whereinthe sodium aluminate solution has a ratio A/C of an aluminaconcentration A (g/liter) to a sodium hydroxide concentration C(g/liter) of 0.45 or less.
 4. The method according to any one of claims1 or 2, wherein the aluminum hydroxide has a percent dissolution, due tothe slurry temperature elevation, of less than 15%, the percentdissolution being represented by:Percent dissolution (%) =C (before temperature elevation)×{A/C (aftertemperature elevation)−A/C (before temperature elevation)}×1.53/slurryconcentration (before temperature elevation)×100, wherein A representsthe alumina concentration (g/liter) of the sodium aluminate solution andC represents the sodium hydroxide concentration (g/liter) of the sodiumaluminate solution.
 5. The method according to any one of claims 1 or 2,wherein the centrifugal force is at least 300 G.
 6. The method accordingto any one of claims 1 or 2, wherein the centrifugal force is applied bymeans of a continuous screw decanter.
 7. The method according to any oneof claims 1 or 2, wherein the slurry temperature elevation is performedusing a double-tube heat exchanger serving as a temperature elevationapparatus employed in the step for elevating the temperature.